The invention relates to a semiconductor device with a semiconductor body comprising an island-shaped first region of a first conductivity type adjoining a surface and adjoining thereto and also adjoining the surface a second region of the second conductivity type which, seen at the surface, surrounds the first region along its entire circumference and forms a pn junction therewith and which is provided with a breakdown voltage raising edge in the form of a first surface zone of the second conductivity type which is provided in the first region and which is situated at such a small distance from the second region that, given a sufficiently high voltage in the reverse-bias direction across the pn junction, the surface zone is entirely situated within the depletion region of the blocked pn junction. The semiconductor body may be of a usual construction with a silicon substrate of the second conductivity type and an epitaxial layer of the first conductivity type provided thereon. Usually, the substrate is of the p type and the epitaxial layer, or epi layer for short, of the n type. Obviously, however, the conductivity types may be interchanged. An island-shaped first region is defined in the epi layer by means of a p-type island insulation zone surrounding the island and forming the said second region of the second conductivity type. In alternative embodiments, the first region is formed, for example, by an n-type implanted zone (well or pocket) provided in a p-type substrate, the second region of the second conductivity type being formed by non-redoped adjoining portions of the substrate. The background of the invention and the invention itself will be described hereinafter in particular with reference to the first embodiment indicated here, but it will be obvious that the invention is not to be limited to embodiments comprising an epi layer.
During operation the pn junction between the island and the insulation zone is reverse-biased for obtaining a good island insulation. The voltage across the junction must be lower than the breakdown voltage of the pn junction. It is known that the breakdown voltage of planar pn junctions is often considerably lower than might be expected on the ground of the doping concentrations. This is due inter alia to local increases in the field strength caused by inter alia surface effects and in particular by the edge curvature of the pn junction.
A semiconductor device of the kind described in the opening paragraph is known from inter alia U.S. Pat. No. 4,750,028. This describes a device in which a number of comparatively weakly doped p-type surface zones are provided along the edge of the island for reducing the field strength where the pn junction intersects the surface between the island and the insulation zone. Although a single ring situated within the depletion region of the pn junction already leads to a considerable improvement of the breakdown voltage, several rings next to one another are preferably used, as also shown in the said U.S. Patent, the outermost ring directly adjoining the insulation zone while the remaining rings are situated at a distance from one another. In particular when several breakdown-voltage-raising rings are used, this configuration requires comparatively much space in the semiconductor body which cannot usually be effectively used for providing circuit elements.